Cancer is caused by uncontrolled and unregulated cellular proliferation. The consequence of this often rapid proliferation is a high level of oxidative stress within the tumour which damages DNA and leads to a much-increased mutation rate. Tumour cells therefore engage and rely heavily upon DNA damage repair mechanisms.
Single-strand breaks (SSBs) are the commonest type of lesion arising in cells and PARG (Poly ADP-ribose glycohydrolase) together with PARP is involved along with a number of other proteins in single strand break repair (SSBR) and another repair mechanism called base excision repair (BER).
One of the earliest events during single strand DNA repair is the binding of PARP (poly ADP-ribose polymerase) to the break and the rapid synthesis of poly ADP-ribose (PAR) on PARP itself. This molecular structure serves as a signal to recruit other DNA repair proteins, initially XRCC1, which will then repair the break (Mortusewicz, Fouquerel et al. 2011). The signal initiated by these PAR chains is short-lived as they are rapidly degraded by the enzyme PAR glycohydrolase (PARG). When PARP is bound to PAR, its catalytic activity is reduced and therefore PARG activity helps to restore PARP to its catalytically active form (Curtin and Szabo 2013).
PARG exists as a single gene with isoforms that reside in the nucleus, mitochondria and cytosol. The only other known protein with glycohydrolase activity is ARH3 which is localised to the mitochondria (Mashimo, Kato et al. 2014). Although, known primarily for its direct role in DNA repair, PARG impacts PAR signalling in splicing, transcriptional and epigenetic pathways (Ji and Tulin 2009) (Le May, Iltis et al. 2012) (Dahl, Maturi et al. 2014) (Guastafierro, Catizone et al. 2013) (Caiafa, Guastafierro et al. 2009).
Cancer cells may become addicted to a specific DNA repair pathway when other mechanisms of DNA repair are non-functional. Tumours carrying mutations in proteins involved in double strand break repair are often more sensitive to PARP inhibitors of SSBR. There is already some evidence that PARG depletion inhibits SSBR and reduces survival of BRCA2-deficient cells (Fathers, Drayton et al. 2012). However, other tumour mutations may give rise to deficiencies in double strand DNA repair mechanisms (so-called “BRCA-ness”) thereby sensitising tumour cells to PARG inhibition.
PARG depletion has been studied in a number of murine and human model systems. Murine cells that are null or depleted for PARG display an increased sensitivity to experimental and clinical DNA damaging agents. However, as deficiency in PARG doesn't sensitise to all agents (e.g. gemcitabine, camptothecin) this suggests a specificity for PARG function with certain pathways of DNA damage repair and chemo- and radiotherapies (Fujihara, Ogino et al. 2009) (Shirai, Fujimori et al. 2013) (Zhou, Feng et al. 2010) (Zhou, Feng et al. 2011).
In humans PARG depletion sensitises lung, cervical and pancreatic cancer cells to γ-irradiation or experimental DNA damaging agents (e.g. hydrogen peroxide, Methylmethanesulfonate) (Ame, Fouquerel et al. 2009) (Nakadate, Kodera et al. 2013) (Shirai, Poetsch et al. 2013).
PARP inhibitors are currently undergoing a raft of clinical trials where the concept of synthetic lethality or chemo-sensitisation is being explored. Clinical resistance to PARP inhibitors has already been described (Drost and Jonkers 2014) (Barber, Sandhu et al. 2013) and therefore there is a requirement that alternative inhibitors targeting the DNA damage repair machinery are found. As PARG depletion leads to reduced rates of SSBR to the same extent as depletion of PARP1, PARG inhibition may provide a therapeutic advantage in PARP inhibitor resistant cells (Fisher, Hochegger et al. 2007). Furthermore, depletion of PARG has been reported to lead to a markedly different gene expression pattern to that of PARP depletion in breast cancer cells (Frizzell, Gamble et al. 2009).
Although current models show that PARG depletion leads to PARP-dependent effects on DNA repair, recent research has shown a mechanistic differentiation from PARP inhibition. Following a genotoxic stimulus depletion of PARG, in contrast to PARP depletion, leads to a drop in NAD levels. This leads to lung cancer cell death that may be as a result of energy failure (Erdelyi, Bai et al. 2009).
Cell permeable PARG inhibitors have been limited to compounds such as Tannic acid or Gallotannin which have questionable specificity for PARG and limited bioavailability (Sun, Zhang et al. 2012) (Fathers, Drayton et al. 2012) (Blenn, Wyrsch et al. 2011).
An object of this invention is to provide specific cell permeable inhibitors of PARG.